Phase-amplitude-frequency measuring system



SePf- 7, 1954 YR. v. RHoADEs ETAL 2,638,726

PHASE-AMPLITUDE-F'REQUENCY MEASURING SYSTEM Filed Jan. l1, 1951 Patented Sept. 7, 1954 u NflT-ao sii-Tieres PATENT o1=rl1clav PHASE-AMPLITUDE-FREQUENCY MEASURING SYSTEM Rex-V. Rhoades, Redondo Beach, Lee C. Keene, ManhattanBeach,A and Frederick Stevens, Long Beach, Calif.,-` assignors to Northrop Aircraft,

Inc.,4 Hawthorne, Calif., a corporation of 'Californ-ia Application January 11, 1951, Serial No. 205,542.'

5 Claims... (Cl. S24-57) This invention relatesto a phase-amplitude measuring device, and more mparticu1arly-to -a means and a method of detecting andmeasuring any'change in phase and/or amplitude between theV input and output of an electrical device-- Itis known thatmanyelectrical devices, such as servomechanisms, electronicamplifiers, autopilotdevices, and the'like, `display a shift in phase relationship, or a change innamplitude, or both, between an alternating signal-voltage `at the-finp-ut tothe device and the alternating voltage appearing at the output of the device. This'phase shift and amplitude change vary with lfrequency of V theapplied input-signal. It is oftendesirable to detect and measure the amount ofphase shift that occurs in an ,electrical device and'tol detect and measure the ratioof amplitudefbetween the input andthe output alternating voltage.

The phase-amplitude measuring` device is usedto obtain sinusoidal responses Vof lelectrome ehancal systems, to obtain phaseand amplitude ratios at'various frequencies. These ratio -measf providea method andA a means whereby-'a differ--y enceA of amplitude betweenthe alternating voltage input and outputmay be detected and measured, since the overall'gain of Vthe'device being tested'at various frequencies is'thus determined.

A further obj ectof the present invention isy to provide. a method and a means whereby thefrequency of the input voltage signals, as applied to the'circutto be tested, may beyaried overa suitable'range in order to obtain data on operate ing characteristics of the circuit under test at various. conditions.

This invention possesses numerous other objects and features, some of whichytogether with the foregoing, will be set forth in the following description of a preferred embodiment of the invention.

Broa'dlyI stated, `the present invention -iscomprised of'alternating voltage generating devices and a vphase and voltage amplitude measuring means. Two alternating voltages are generated f whose amplitude, frequency, and phase relationship areknown; one voltage traversesthe-circuit under testand thenis'compared to the'other voltage to determinea shift inphase. Theoutput voltage' from the circuitl undertest I is I also compared toca voltage ef known-amplitude VIn -theY drawing, the-ligure is la perspective,l diagrammatic illustration of the preferred em- ,2 bodiment of the invention'together Witha wiring diagram Aof certain parts thereof;

A constant frequency alternating voltage isapplied to a synchronous motor- 2- through-motorleads i. rlhe motor 2 is mountedona main sup--y portframe Iiaof the measuring devicaandhas a rotor-2a rotatably connected to a disk 5 through drivegears-i and drive sha-ftd. Afball 'l yis so dsposedby a ball keeper E that it is always in contact with the surface of the disk E :and the surface` of a drum I3, to thereby provide .a mechanical friction drive betweenthe disk and the drum. A follower I5 on the-edge of the ball keeper G-engages a helical cut Ita on the surface of a cylindrical cam 8;y therefore,-rotation of the cylindrical cam8 results in a translation motion of the ball keeper BA and ball 'i such thatthe-point of contact betweenthe disk .'i and the ball 'i willI trace a. straight line fromthe center ofthe disk- 5 to its rim, parallel to the 4axis of `thedrum I3. The rotation of the cylindrical cam 8 is realized by rotating a speed control knob l I If, which is connected to cylindrical camb-y means of cam shafts aand cam gears Il).

The rotational output of the drum I3 is trans-- mitted through drum-shafts I4, drumlgears It,

and two bevel gears-IlS to a sinusoidal potentiometer shaft I8. Positioned-on and rotating with the sinusoidal potentiometer shaft I8 are four vslip rings I9 insulatedfrom the shaft, and two sinusoidal potentiometercoil'assemblies 23.

The sinusoidal potentiometer coils 23 are each energized by the voltage at the output of a firstvoltage source 2I (preferably D. C.) as follows: Source leads 22 connected tot four stationary brushes 20,- wh-ich bear respectively onfthe slip rings I9, and potentiometer leads 3d connected betweeneachslip ring I9 and opposite ends of its respective potentiometer coil 23, afford a means whereby a continuous electrical eircuit'is maintained between theoutput of the irstvoltage source 2l and the rotating coils A23-01" the sinusodial zpotentiometers. A means, such as rheostat knob 2d, is provided within first Voltage source ZI ntol Vary the input'voltage applied to coils 23. This voltage varying means is so calibrated that the amplitudeof the alternating voltage at the `potentiometer output can be selected over a wide, known range of values.

Eachlcoil "Sbonsists of a comparatively high resistance wire wound in a linear fashion on a stiff card coliform 23a. The `coil wire is .normally insulated, but the insulation is removed from one, exposed, face-ofthe vcoil in order thatfa pair of brushes 25 may make van electrical' contact with.

each coil.

The brushes 25' are positioned equi-distantly apart and are so located that they are in contact with the coil-23 and trace a circular path on the coil 23 as thevcoil rotates. tiometer brush-holder 28 is permanently attached One sinusoidal.v poten,

to the frame of the device. Another sinusoidal potentiometer brush-holder 29 is capable of being rotated about an axis parallel to the potentiometer shaft I8 by rotation of a phase control knob 31, brush-holder shaft 36, and worm 35.

The thread of the worm meshes with worm gear teeth 35a on the periphery of the rotatable brush-holder 29 such that a rotation of phase control knob 31 is transmitted to worm 35 through brush-holder shaft 36 and thereafter causes a rotational motion of the movable sinusoidal potentiometer brush-holder 29 about an axis perpendicular to the axis of rotation of the worm 35 and coaxial with potentiometer shaft I8.

Output leads 3| from the brushes 25 of the movable brush-holder 29 are applied to one den lection circuit of an oscilloscope 39, such as through horizontal input 33, for example. The output of brushes 25 of the xed brush-holder 28 is applied to a circuit to be tested 4I through test circuit input leads 32 to the test circuit input 42. A test circuit output at 43 is taken from the circuit to be tested 4I and is applied to the other deflecting circuit of oscilloscope 39 through vertical input 34 by means of test circuit output leads 44. Another output is taken from the circuit to be tested 4I at output 43 and is applied to input connections 68 of a pair of slide-back voltmeters 6I through voltmeter leads 62.

A second voltage source 45 applies a known D. C. voltage to the slide-back voltmeter assembly GI through wires 48. Center-output resistors 41 effectively ground the center of potentiometer-s 48 and 49. By grounding the center of potentiometers 48 and 49 the reversing of voltmeter leads 62 at the input connections 60 of the slide-back voltmeter assembly 6I to obtain the correct positive and negative peak voltage reading on Voltmeters 8| is obviated, since the movable contact of the potentiometers 48 and 49 may be moved from a zero-reading central position to values in either direction. The movable contact on potentiometers 48 and 49 can be adjusted until the voltage at the movable contact matches the voltage at the input 69, which will be indicated by no current flowing through ammeters 55 and 5|. The diode combinations 52 and 54 and the diode combinations 53 and 55 provide a sharp rectification of the input voltage at 69, thereby enabling the ammeter 50, for example, to detect the negative peak voltage and the ammeter 5I to detect the positive peak voltage of the output of the circuit to be tested 4I. The resistor-switch combinations 56 and 58 and the resistor-switch comn binations 51 and 59 provide a meter protection and a sensitivity means for the voltmeters in that the initial readings of the ammeters and 5I are made with the resistors and 51 in their respective circuits, then the resistors are short circuited by closing switches 58 and 59 to obtain a more precise measurement.

Assume that the cir-cuit to be tested, 4Iy has been selected and that its test frequency and input voltage are established. The speed control knob I I is then adjusted until its pointer matches the desired frequency, as indicated on a frequency scale I2. In adjusting control knob II, the cylindrical cam 8 is rotated to thereby translate ball keeper 6 and ball 1 along a path parallel to the axis of drum I3 to a new position with respect to the center of the disk 5. The synchronous motor 2 is energized by applying power to motor leads I and torque is applied from the motor 2 to the coils 23 of the sinusoidal potentiometer-s by means of shafts 3, I4 and I8, gears 4, I6 and I1, and by means of the ball and disk speed-changer. The coil assemblies 23 of the sinusoidal potentiometers are then rotating at a speed as determined by the ball and disk speed-changer such that the desired frequency of the alternating voltage appearing a't the output leads 3| and test circuit input leads 32 for operating the circuit to be tested 4| is obtained.

The sinusoidal potentiometers are disposed within this particular embodiment such that when the pointer of phase control knob 31 is set opposite zero phase shift on a phase scale 38 and when the two coil assemblies 23 are energized and rotating, the alternating voltage outputs through leads 32 and 3| are in phase. The only means whereby a difference of phase can be realized between leads 3| and 32 is by a rotation of the sinusoidal potentiometer brush-holder 29 and the brushes attached thereto with respect to the sinusoidal potentiometer brush-holder 28. The phase scale 38 has calibrated markings so positioned thereon that a reading between the pointer of phase control knob 31 and scale 38 affords a measure of the phase relationship existing between the alternating Voltage in leads 3| and leads 32.

The output voltage from leads 3| is applied directly to one pair of deection plates of oscilloscope 39 while the output voltage from leads 32 is applied to the circuit to be tested 4I from which an output voltage is taken to be applied to the other pair of deflection plates of oscilloscope 39. Thus, assuming the frequency of the alternating voltage is not altered in traversing the circuit under test 4|, i. e., the frequencies at inputs 33 and 34 of oscilloscope 39 are identical, the Lissajou pattern appearing on screen 40 of oscilloscope 39 will be an ellipse with circles and straight lines as limiting conditions. If the input alternating voltages that are applied to oscilloscope 39 at inputs 33 and 34 are in phase or are 180 out of phase, a straight line Lissaj ou pattern will appear on screen 48; the straight line for the in phase condition being distinguishable from the 180 out of phase condition by the difference of direction it makes on the screen 48. The direction of the in phase straight line on the oscilloscope screen 40 may be quickly determined by eliminating oscilloscope input leads 44 and making a direct connection between the output of leads 32 and the input to oscilloscope 39 at 34.

When the circuit to be tested 4| produces a phase shift in the alternating voltage between its input 42 and its output 43 an elliptical Lissajou pattern will appear on screen 4D. If the length of the minor axis of the ellipse equals the length of the major axis, i. e., a circle, then the phase shift has been equal to In order to determine whether a leading or lagging phase shift is occurring in the circuit to be tested, a rough calculation of the phase shift may be made prior to utilizing the present invention to determine the phase shift. The rough calculation of the phase shift may be made for the circuit to be tested by using a frequency such that the calculations are made easy. The method of operation is then to adjust speed control knob I I until the pointer matches the value of the frequency of the calculations on the frequency scale I2, then readjust speed control knob II until the pointer matches the frequency of desired operation on the scale I2, meanwhile carefully noting the characteristic of the change of the Lissajou pattern on the screen 4U to obtain the change of phase shift between the frequency of the calculations and the frequency of desired o-peration. By adjusting the phase control knob 31 until the trace area on the oscilloscope screen 40 is reduced to zero at the position indicating an in phase condition of the voltages in leads 3l and 44, the phase shift, caused in the test circuit 4| at the particular control frequency being applied, can be rea'd directly from the calibrated phase scale 38.

Initially, the switches 58 and 59 of the slide-- back voltmeter assembly 6| are open. The sliding contacts of the potentiometers 48 and 49 are then adjusted until no current is flowing through ammeters 59 and 5 I. The condition cf no current flowing in ammeters 59 and 5l denotes that the voltage at the sliding contacts in potentiometers 48 and 49 equals the voltage of the positive and the negative peaks of the alternating voltage at the input B9. Calibrated scales (not shown) in conjunction with control knobs rotatably connected to the sliding contacts of potentiometers 48 and 49 operate to give a visual indication of the value of the peak negative and positive voltages at input 6U with respect to ground. Switches 58 and 59 are then closed, thereby short circuiting meter protector resistors 56 and 51 and increasing the accuracy of measurement of the peak voltages. The sliding contacts are readjusted until a nocurrent flow condition again exists in the ammeters 59 and 5I, to give a final value for the peak voltages. The peak-to-peak amplitude of the alternating voltage at the circuit output 43 is then obtainable from the positive and negative peak voltages. The ratio of output to input voltage of the circuit to be tested 4I is then obtained by comparing the output amplitude determined above with the amplitude at the circuit input 42, the latter being a known value since the input voltage to the potentiometer coils 23 is known.

Thus it is seen that the present invention provides a means of quickly obtaining the phase shift and voltage gain of an electrical network. At the low frequencies for which this invention is best suited, the accuracies attainable are within 11:0.2 degree in phase shift determination, and within 0.5% of the input voltage in the amplitude gain determination. Test frequencies can be set with an accuracy of 0.2%. While specifically adapted for low frequency work, this invention may be used at other, higher frequencies, or any number of variations in the basic method may easily be made, as Will be obvious to those persons skilled in the art.

From the above description it wil1 be apparent that there is thus provided a device of the character described possessing the particular features of advantage before enumerated as desirable, but which obviously is susceptible of modification in its form, proportions, detail construction and arrangement of parts without departing from the principle involved or sacrificing any of its advantages.

While in order to comply with the statute, the invention has been described in language more or less specific as to structural features, it is to be understood that the invention is not limited to the specific features shown, but that the means and construction herein disclosed comprise a preferred form of putting the invention into effect, and the invention is, therefore, claimed in any of its forms or modifications within the legitimate and valid scope of the appended claims.

What is claimed is:

1. In a phase-shift determining system for low frequency applications, means for generating a pair of phase-shifted alternating voltages comprising a voltage source, a rotatable rigid shaft, driving means for rotating said shaft, a pair of sinusoidal potentiometer coils xed to said shaft, sliding contact means electrically connecting said potentiometer coils across said voltage source, a first pair of potentiometer pick-off brushes arranged at the end of one potentiometer coil to slidably bear against said coil and trace a closed path thereon as said coils rotate, a stationary brush holder holding said first pair of brushes fixed, a second pair of potentiometer brushes arranged at the end of the other potentiometer coil to slidably bear against said other coil and trace a closed path thereon as said coils rotate, a rotatable brush holder carrying said second pair of brushes, means for adjusting said rotatable brush holder about the center line of said shaft, means for connecting a first output circuit to said rst pair of brushes, and means for connecting a second output circuit to said second pair of brushes, whereby measurable phase shift can be made between the waveforms in said output circuits without affecting the relative amplitude or purity of said waveforms.

2. Apparatus in accordance with claim 1 wherein said driving means comprises a constant speed motor, continuously variable speed changing means connected between said motor and said shaft, and a calibrated control element operatively connected to said speed changing means, the calibrations of said control element registering the frequency in said iirst and second output circuits as produced by rotation of said sinusoidal potentiometer coils.

3. Apparatus in accordance with claim 2 wherein said continuously variable speed changing means comprises a ball and disc integrator type adjustable transmission, including a cylinder in driven relationship with the ball, whereby very low shaft speeds are obtained to produce accurate stable output frequencies in the neighborhood of .01 C. P. S. to 2 C. P. S.

4. Apparatus in accordance with claim 1 wherein said potentiometer coils are identical and are connected in parallel with each other across said Voltage source, whereby said waveforms are respectively equal in amplitude.

5. Apparatus in accordance with claim 1 including phase changing and indicating means which comprises a calibrated manual control member directly connected to move said rotatable brush-holder to turn said second pair of brushes relative to said first pair of brushes, the calibrations on said control member registering Zero when the voltage of said second output circuit is in phase with the voltage of said first output circuit as said shaft is rotated.

References Cited in the file of this patent UNITED STATES PATENTS Ingalls May 6, 1952 

